Automatic flicker detection and correction apparatus and method in a video capture device

ABSTRACT

To automatically detect and correct flickers in a video capture device, a frame number generator receives a plurality of frames and generates frame numbers. A storage device stores brightness values of partial pixels of a frame numbered 1. An extractor extracts brightness values of partial pixels of a frame numbered N. A difference summation device computes differences of the brightness values of the partial pixels of the frames numbered 1 and numbered N and sums up the differences to generate a brightness difference summation signal. A flicker detector detects a banding value in the brightness difference summation signal. A flicker corrector uses the banding value and the frame numbers to find a fixed frequency flicker effect in the frames, and generates a flicker correction signal when the fixed frequency flicker effect in the frames is found.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technical field of image processingand, more particularly, to an automatic flicker detection and correctionapparatus and method in a video capture device.

2. Description of Related Art

Typically, the illuminants have different frequency flickers due to thedifferent AC frequencies in different countries. The AC frequencies areessentially divided into two types: 60 Hz such as in Taiwan where alight has a period of 1/120 second per flicker, and 50 Hz such as inChina where a light has a period of 1/100 second per flicker. Forcapturing an image by a CMOS video capture device under such anenvironment, every horizontal line of an image frame has the uniformexposure amount when the exposure time is set to a multiple of oneflicker period of environment lighting, and no flicker appears on theimage frame. However, flicker may occur since the exposure amount onevery line of the image frame is not uniform when the exposure time isnot equal to a multiple of one flicker period of environment lighting.

To overcome the flicker on the image captured by the CMOS video capturedevice, US Patent Application Publication No. 2007/0153094 has disclosedan “Automatic flicker correction in an image capture device”, which usesthe differences of the total brightness of every horizontal line of twoadjacent frames to detect fixed frequency flicker, and the detectedfrequency to correct the exposure time of the image capture device whenthe fixed frequency flicker is detected.

However, such a correction cannot detect the original frequency afterthe differences of the total brightness of every line, which are smallor even almost zero when the flicker effect on the image caused byillumination source is very light, are passed through a low pass filter(LPF). In addition, for finding the differences of the total brightnessof every line of the adjacent frames, the total brightness of every lineof the first frame is temporarily stored in the memory, and if the size(640×480) of a popular VGA frame is considered, the memory size requiredfor storing 480 brightness values (in the way of 18 bits per brightnessvalue) is 8640 bits (=1080 bytes). After the differences of the totalbrightness of all lines are obtained and passed through the LPF, thedifferences are further passed through a derivative calculator, whichrequires a huge amount of computation and thus takes much time to affectthe normal display rate.

Therefore, it is desirable to provide an improved detection andcorrection apparatus to mitigate and/or obviate the aforementionedproblems.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an automatic flickerdetection and correction apparatus and method in a video capture device,which can effectively detect an oblique line in a chromatic mixturesignal.

In accordance with a feature of the invention, there is provided anautomatic flicker detection and correction apparatus in a video capturedevice, which includes a frame number generator, a storage device, anextractor, a difference summation device, a flicker detector and aflicker corrector. The frame number generator receives a plurality offrames and accordingly generates frame numbers. The storage device isconnected to the frame number generator in order to store brightnessvalues of partial pixels of a frame numbered one. The extractor isconnected to the frame number generator in order to extract brightnessvalues of partial pixels of a frame not numbered one. The differencesummation device is connected to the storage device and the extractor inorder to compute differences of the brightness values of the partialpixels of the frames numbered one and not numbered one and to sum thedifferences to thereby generate a brightness difference summationsignal. The flicker detector is connected to the difference summationdevice in order to detect a banding value in the brightness differencesummation signal. The flicker corrector is connected to the flickerdetector and the frame number generator in order to use the bandingvalue and the frame numbers to find a fixed frequency flicker effect inthe frames, and to generate a flicker correction signal when the fixedfrequency flicker effect in the frames is found.

In accordance with another feature of the invention, there is provided avideo capture device with automatic flicker detection and correction,which includes an image sensor, an image processor, an automaticdetection and correction unit and an image sensor controller. The imagesensor captures a plurality of frames. The image processor is connectedto the image sensor in order to perform an image processing on theframes to thereby produce a plurality of processed frames. The automaticflicker detection and correction apparatus is connected to the imageprocessor in order to receive the processed frames, compute brightnessdifferences of partial pixels of a processed frame numbered one andmultiple processed frames not numbered one, sum the brightnessdifferences to accordingly generate a brightness difference summationsignal, detect a banding value in the brightness difference summationsignal to thereby determine whether a fixed frequency flicker effectexists in the processed frames, and generate a flicker correction signalwhen the fixed frequency flicker effect exists in the processed frames.The image sensor controller is connected to the image sensor and theautomatic flicker detection and corrector apparatus in order to use theflicker correction signal to adjust operating parameters of the imagesensor.

In accordance with a further feature of the invention, there is providedan automatic flicker detection and correction method in a video capturedevice, which includes: (A) numbering a plurality of frames toaccordingly generate frame numbers; (B) storing brightness values ofpartial pixels of a frame numbered one; (C) extracting brightness valuesof partial pixels of a frame not numbered one; (D) calculatingdifferences of the brightness values of the partial pixels of the framesnumbered one and not numbered one, and summing the differences toaccordingly generate a brightness difference summation signal; (E)detecting a banding value in the brightness difference summation signal;and (F) using the banding value and the frame numbers to find a fixedfrequency flicker effect in the frames, and generating a flickercorrection signal to accordingly correct an image sensor of the videocapture device when the fixed frequency flicker effect in the frames isfound.

Other objects, advantages, and novel features of the invention willbecome more apparent from the following detailed description when takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a video capture device with automaticflicker detection and correction in accordance with the invention;

FIG. 2 is a block diagram of an automatic detection and correctionapparatus in accordance with the invention;

FIG. 3 is a control flowchart of a flicker detector in accordance withthe invention;

FIG. 4 is a control flowchart of a flicker corrector in accordance withthe invention;

FIG. 5 is a flowchart of an automatic flicker detection and correctionmethod in a video capture device in accordance with the invention; and

FIG. 6 is a schematic graph of a waveform of a brightness differencesummation signal in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a block diagram of a video capture device 100 with automaticflicker detection and correction in accordance with the invention. InFIG. 1, the device 100 includes an image sensor 110, an image processor120, an automatic flicker detection and correction apparatus 130, and animage sensor controller 140.

The image sensor 110 captures a plurality of frames. The image sensor110 can be a CMOS or CCD array image sensor.

The image processor 120 is connected to the image sensor 110 in order toperform an image processing on the frames to thereby produce a pluralityof processed frames.

The automatic flicker detection and correction apparatus 130 isconnected to the image processor 120 in order to receive the processedframes, compute brightness differences of partial pixels of a processedframe numbered one and multiple processed frames not numbered one, sumup the brightness differences to accordingly generate a brightnessdifference summation signal, detect a banding value in the brightnessdifference summation signal to thereby determine whether a fixedfrequency flicker effect exists in the processed frames, and generate aflicker correction signal when the fixed frequency flicker exists in theframes.

The image sensor controller 140 assumes that a background light sourceis at 60 Hz, and accordingly sets a shutter time and a frame rate of theimage sensor 110 to 1/120 sec. and 30 frame/sec., respectively, when thevideo capture device 100 is powered on.

When the video capture device 100 is powered on, the image processor 120adjusts a gain of the image sensor 110 through the image sensorcontroller 140 in order to extract the frames. The image processor 120performs an auto-focusing (AF) process based on the frames in order tohelp the image processor 12 to clearly extract the images.

When the image sensor controller 140 receives the flicker correctionsignal, it means that the background light source (such as a fluorescentlight) does not use an AC frequency of 60 Hz, and in this case theshutter time of the image sensor is adjusted to a multiple of 1/100second.

FIG. 2 is a block diagram of the automatic flicker detection andcorrection apparatus 130 in accordance with the invention. In FIG. 2,the apparatus 130 includes a frame number generator 210, a storagedevice 220, an extractor 230, a difference summation device 240, aflicker detector 250, and a flicker corrector 260.

The frame number generator 210 is connected to the image processor 120in order to receive the processed frames and generate the frame numbersbased on the processed frames in sequence.

The storage device 220 is connected to the frame number generator 210 inorder to store the brightness values of the partial pixels of aprocessed frame numbered one (such as the first frame).

The extractor 230 is connected to the frame number generator 210 inorder to extract the brightness values of the partial pixels of eachprocessed frame not numbered one (such as the processed frame with aframe number greater than one).

The positions for storing the partial pixels of the processed framenumbered one in the storage device 220 correspond to the positions ofthe partial pixels of each processed frame not numbered one that areextracted by the extractor 230.

The difference summation device 240 is connected to the storage device210 and the extractor 220 in order to compute differences of thebrightness values of the partial pixels of the processed frames numberedone and not numbered one, and to sum up the differences to therebygenerate a brightness difference summation signal Y_Diff_Sum(n), whichis expressed as follows.

${{{Y\_ Diff}{\_ Sum}(n)} = {\sum\limits_{x = 1}^{k}\; \left( {{Y_{x}^{n} - Y_{x}^{1}}} \right)}},{k \leq 1000},$

where Y_(x) ¹ indicates the brightness values of the partial pixels ofthe processed frame numbered one, Y_(x) ^(n) indicates the brightnessvalues of the partial pixels of the processed frame not numbered one forN is a positive integer greater than one, and k indicates a number ofthe partial pixels.

The flicker detector 250 is connected to the difference summation device240 in order to detect a banding value, denoted as banding_value, in thebrightness difference summation signal.

FIG. 3 is a control flowchart of the flicker detector 250 in accordancewith the invention. As shown in FIG. 3, step S310 compares thebrightness difference summation signal Y_Diff_Sum(n) with a previoussignal. When the signal Y_Diff_Sum(n) is greater than the previoussignal, step S320 determines a status to be up or not. When status=up,it indicates that the signal Y_Diff_Sum(n) is at a rising edge of awaveform, and in this case step S330 adds one to a variable ofwavelength.

Conversely, when status=up is not found, it indicates that the signalY_Diff_Sum(n) is at a position just out of a valley, and in this casestep S340 sets status=up and further determines a variable ofpre_wavelength to be zero (initial value) or not. When pre_wavelength=0,it indicates that the first valley of the signal Y_Diff_Sum(n) is notformed, and in this case pre_wavelength=wavelength is set. Whenpre_wavelength=0 is not found, an absolute value of subtracting thevariable pre_wavelength from the variable wavelength is taken, and thebanding value banding_value plus the absolute value is set. Next, thevariable of wavelength is initialized by setting the variable ofwavelength to one.

Next, step S350 outputs the banding value banding_value.

In step S310, when the signal Y_Diff_Sum(n) is equal to the previoussignal, it indicates that the signal Y_Diff_Sum(n) is at a peak, and inthis case the variable of wavelength is added by one (step S360).

In step S310, when the signal Y_Diff_Sum(n) is smaller than the previoussignal, it further determines whether the status is down (step S370).When status=down, it indicates that the signal Y_Diff_Sum(n) is at afalling edge of the waveform, and in this case step S380 adds one to thevariable of wavelength. Conversely, when status=down is not found, itindicates that the signal Y_Diff_Sum(n) is at a position just out of apeak, and in this case step S390 sets status=down and adds one to thevariable of wavelength.

The flicker corrector 260 is connected to the flicker detector 250 andthe frame number generator 210 in order to use the banding valuebanding_value and the frame numbers to find a fixed frequency flickereffect in the processed frames and to generate a flicker correctionsignal, denoted as flicker_correction, when the fixed frequency flickereffect in the frames is found.

FIG. 4 is a control flowchart of the flicker corrector 260 in accordancewith the invention. As shown in FIG. 4, step S410 determines whether theframe number is equal to a predetermined value N. When the frame numberis equal to the value N, step S420 determines whether the banding valuebanding_value is smaller than a threshold Thr. When the banding valuebanding_value is smaller than a threshold Thr, it indicates that thewaveform forming the signal Y_Diff_Sum(n) has almost identicalwavelengths, which means that the fixed frequency flicker effect appearsin the frames extracted. Accordingly, since the flicker effect appears,it is incorrect to assume that a background light source is at 60 Hzwhen the video capture device 100 is powered on, and thus step S440 setsa variable of light_freq to 50 Hz.

In step S420, when the banding value banding_value is not smaller than athreshold Thr, it indicates that the waveform forming the signalY_Diff_Sum(n) has no fixed frequency flicker effect, and accordinglystep S430 sets the variable of light_freq to 60 Hz because it is correctto assume that a background light source is at 60 Hz.

In step S450, when the variable of light_freq has to be changed from 60Hz to 50 Hz or from 50 Hz to 60 Hz, the signal flicker_correction isgenerated.

In step S410, when the frame number is not equal to N, step S410 isre-started at next frame.

The flowcharts of FIGS. 3 and 4 can be implemented easily by a personskilled in the art with a hardware description language such as SystemC, Verilog or VHDL, and thus a detailed description is deemedunnecessary.

FIG. 5 is a flowchart of an automatic flicker detection and correctionmethod in the video capture device 100 in accordance with the invention.As shown in FIG. 5, step S510 initializes the image sensor 110, whichsets the shutter time of the image sensor 110 to 1/120 sec. and a framerate of the image sensor 110 to 30 frame/second (FPS), and adjusts again of the image sensor 110 in order to extract the frames.

Step S520 numbers the frames in order to generate the frame numberscorresponding to the frames.

Step S530 stores the brightness values of partial pixels of a framenumbered one.

Step S540 extracts the brightness values of partial pixels of a framenot numbered one.

The positions for storing the partial pixels of the frame numbered onein step S530 correspond to the positions of the partial pixels of theframe not numbered one that are extracted in step S540.

Step S550 computes the differences of the brightness values of thepartial pixels of the frames numbered one and not numbered one toaccordingly generate the brightness difference signals corresponding tothe differences.

Step S560 sums up the brightness difference signals to thereby generatea brightness difference summation signal Y_Diff_Sum(n), which isexpressed as follows.

${{{Y\_ Diff}{\_ Sum}(n)} = {\sum\limits_{x = 1}^{k}\; \left( {{Y_{x}^{n} - Y_{x}^{1}}} \right)}},{k \leq 1000},$

where Y_(x) ¹ indicates the brightness values of the partial pixels ofthe processed frame numbered one, Y_(x) ^(n) indicates the brightnessvalues of the partial pixels of the processed frame not numbered one forN is a positive integer greater than one, and k indicates a number ofthe partial pixels.

FIG. 6 is a schematic graph of the waveform of the brightness differencesummation signal Y_Diff_Sum(n) in accordance with the invention, where avertical axis indicates the brightness difference summation signalY_Diff_Sum(n) and a horizontal axis indicates the frame number.

Step S570 detects a banding value banding_value in the brightnessdifference summation signal Y_Diff_Sum(n). Step S580 is based on thebanding value banding_value and the frame number N to determine whethera fixed frequency flicker effect exists in the frames, i.e., to find afixed frequency flicker effect in the frames. When the fixed frequencyflicker effect exists in the frames, a flicker correction signalflicker_correction is generated to correct the image sensor 110 of thevideo capture device 100 and further adjust the shutter time of theimage sensor 110 as a multiple of 1/100 second.

While the exposure time of the shutter (the shutter time) of the imagesensor 110 is set to 1/120 second and the video display rate (the framerate) is set to 30 frames per second (FPS), a video frame presents asignificant flicker effect when the environment light is operated at 50Hz. Such a flicker effect results from the fixed frequency andwavelength. However, when the environment light is operated at 60 Hz,the video frame does not present any flicker with identical frequencyand wavelength.

While the exposure time of the shutter (the shutter time) of the imagesensor 110 is set to 1/120 second and the video display rate (the framerate) is set to 30 frames per second (FPS), every line on the videoframe has different exposure amount to different frames. Thus, the totalbrightness of every line shows flickers, and the number of passed framesrequired for each flicker is fixed. Namely, on the basis of thebrightness of every line in a certain frame and after one bright anddark alternation, the number of passed frames at both a next time and afurther next time back to the identical or approximate brightness isidentical or approximate, i.e., the brightness of every line can reacharound the original value after a fixed number of frames are passedevery time.

Upon such an effect, the invention starts at a certain frame (such asFrame 1), and the brightness values of every line of Frame 1 and a frame(such as Frame N) after Frame 1 at corresponding positions aresubtracted and taken an absolute operation. The absolute values areadded to find a sum of the brightness values between Frame N andFrame 1. On the basis of Frame 1, such an operation is performed on thesubsequent frames to thus find successive tens of brightness differencesums. When the flicker effect caused by a light source is too weak, thesuccessive tens of brightness difference sums are very close to therebylead to a wrong determination. To overcome this, the invention furtheruses the sum of the absolute values of the brightness differencesbetween corresponding pixels of two frames such as Frame N and Frame 1.Since every pixel of Frame 1 is specially selected, the pixels can beregarded as the best test points. Thus, even when the flicker effect istoo weak, the obtained values corresponding to the pixels can be used toeasily detect the flickering frequency.

As shown in FIG. 6, when the number of passed frames in every bright anddark alternation is fixed or almost the same, it is determined that theenvironment light source is at 50 Hz, and otherwise at 60 Hz.

Two exposure time tables can be found in the image sensor controller140: one for n/120 second (n as a positive integer) suitable for anoperation environment with an AC frequency of 60 Hz; the other for n/100second suitable for an operation environment with an AC frequency of 50Hz. Accordingly, the invention can automatically detect the AC frequencyin a current operation environment as 60 Hz or 50 Hz. The exposure timetable based on n/120 second is applied for 60 Hz, and the table based onn/100 second is applied for 50 Hz.

In view of the forgoing, it is known that the invention requires verylow computation in detection of the flicker frequency, without the needof the LPF and the derivative calculator that involve multipliers anddividers and relatively increase the IC area and the cost, as cited inthe prior art. Further, in the invention, the frame rate on a videodisplay is not reduced since the required computation is very low, andonly 1 KB and below temporary memory is required for the video display,which relatively reduces the cost.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that many other possiblemodifications and variations can be made without departing from thespirit and scope of the invention as hereinafter claimed.

1. An automatic flicker detection and correction apparatus in a videocapture device, comprising: a frame number generator for receiving aplurality of frames and accordingly generating frame numbers based onthe frames in sequence; a storage device connected to the frame numbergenerator to store brightness values of partial pixels of a framenumbered 1; an extractor connected to the frame number generator toextract brightness values of partial pixels of a frame numbered N,wherein N is an integer greater than 1; a difference summation deviceconnected to the storage device and the extractor to compute differencesof the brightness values of the partial pixels of the frames numbered 1and N and to sum up the differences to thereby generate a brightnessdifference summation signal; a flicker detector connected to thedifference summation device to detect a banding value in the brightnessdifference summation signal; and a flicker corrector connected to theflicker detector and the frame number generator to use the banding valueand the frame numbers to find a fixed frequency flicker effect in theframes and to generate a flicker correction signal for correcting animage sensor of the video capture device when the fixed frequencyflicker effect in the frames is found.
 2. The apparatus as claimed inclaim 1, wherein positions for storing the partial pixels of the framenumbered 1 in the storage device correspond to positions of the partialpixels of the frame numbered N that are extracted by the extractor. 3.The apparatus as claimed in claim 2, wherein the brightness differencesummation signal generated by the difference summation device isexpressed as:${{{Y\_ Diff}{\_ Sum}(n)} = {\sum\limits_{x = 1}^{k}\; \left( {{Y_{x}^{n} - Y_{x}^{1}}} \right)}},{k \leq 1000},$where Y_(x) ¹ indicates the brightness values of the partial pixels ofthe frame numbered 1, Y_(x) ^(n) indicates the brightness values of thepartial pixels of the frame numbered N, and k indicates a number of thepartial pixels.
 4. A video capture device with automatic flickerdetection and correction, comprising: an image sensor for capturing aplurality of frames; an image processor connected to the image sensor toperform an image processing on the frames to thereby produce a pluralityof processed frames; an automatic flicker detection and correctionapparatus connected to the image processor for receiving the processedframes, computing brightness differences of partial pixels of aprocessed frame numbered 1 and multiple processed frames numbered N,summing up the brightness differences to accordingly generate abrightness difference summation signal, detecting a banding value in thebrightness difference summation signal to thereby determine whether afixed frequency flicker effect exists in the processed frames, andgenerating a flicker correction signal when the fixed frequency flickerexists in the frames, wherein N is an integer greater than 1; and animage sensor controller connected to the image sensor and the automaticflicker detection and corrector apparatus to use the flicker correctionsignal to adjust operating parameters of the image sensor.
 5. The deviceas claimed in claim 4, wherein the image sensor controller sets ashutter time of the image sensor to 1/120 second when the video capturedevice is powered on.
 6. The device as claimed in claim 5, wherein theimage sensor controller sets a frame rate of the image sensor to 30frames/second when the video capture device is powered on.
 7. The deviceas claimed in claim 6, wherein the image sensor controller adjusts again of the image sensor when the video capture device is powered on,thereby extracting the frames.
 8. The device as claimed in claim 6,wherein the image sensor controller changes the shutter time of theimage sensor into a multiple of 1/100 second when the flicker correctionsignal is received.
 9. The device as claimed in claim 4, wherein theautomatic flicker detection and correction apparatus comprises: a framenumber generator for receiving the processed frames and accordinglygenerating frame numbers based on the processed frames in sequence; astorage device connected to the frame number generator to storebrightness values of the partial pixels of the processed frame numberedone; an extractor connected to the frame number generator to extractbrightness values of partial pixels of each processed frame numbered N,wherein N is an integer greater than 1; a difference summation deviceconnected to the storage device and the extractor to compute differencesof the brightness values of the partial pixels of the processed framenumbered 1 and each processed frame numbered N and sum up thedifferences to thereby generate a partial brightness differencesummation signal; a flicker detector connected to the differencesummation device to detect the banding value in the brightnessdifference summation signal; and a flicker corrector connected to theflicker detector and the frame number generator to use the banding valueand the frame numbers to find a fixed frequency flicker effect in theprocessed frames and to generate the flicker correction signal when thefixed frequency flicker effect in the processed frames is found.
 10. Thedevice as claimed in claim 9, wherein positions for storing the partialpixels of the processed frame numbered 1 in the storage devicecorrespond to positions of the partial pixels of the processed framenumbered N that are extracted by the extractor.
 11. The device asclaimed in claim 10, wherein the partial brightness difference summationsignal generated by the difference summation device is expressed as:${{{Y\_ Diff}{\_ Sum}(n)} = {\sum\limits_{x = 1}^{k}\; \left( {{Y_{x}^{n} - Y_{x}^{1}}} \right)}},{k \leq 1000},$where Y_(x) ¹ indicates the brightness values of the partial pixels ofthe processed frame numbered 1, Y_(x) ^(n) indicates the brightnessvalues of the partial pixels of the processed frame numbered N, and kindicates a number of the partial pixels.
 12. An automatic flickerdetection and correction method in a video capture device, comprisingthe steps: (A) numbering a plurality of frames to accordingly generateframe numbers; (B) storing brightness values of partial pixels of aframe numbered 1; (C) extracting brightness values of partial pixels ofa frame numbered N, wherein N is an integer greater than 1; (D)calculating differences of the brightness values of the partial pixelsof the frames numbered 1 and N, and summing the differences toaccordingly generate a brightness difference summation signal; (E)detecting a banding value in the brightness difference summation signal;and (F) using the banding value and the frame numbers to find a fixedfrequency flicker effect in the frames, and generating a flickercorrection signal to accordingly correct an image sensor of the videocapture device when the fixed frequency flicker effect in the frames isfound.
 13. The method as claimed in claim 12, wherein step (F) is basedon the flicker correction signal to change a shutter time of the imagesensor into a multiple of 1/100 second.
 14. The method as claimed inclaim 13, further comprising the steps before step (A): (A1) setting theshutter time of the image sensor to 1/120 second; (A2) setting a framerate of the image sensor to 30 frames per second (FPS); and (A3)adjusting a gain of the image sensor to thereby extract the frames. 15.The method as claimed in claim 12, wherein positions for storing thepartial pixels of the processed frame numbered one in step (B)correspond to positions of the partial pixels of the processed framenumbered N that are extracted in step (C).
 16. The method as claimed inclaim 13, wherein the brightness difference summation signal isexpressed as:${{{Y\_ Diff}{\_ Sum}(n)} = {\sum\limits_{x = 1}^{k}\; \left( {{Y_{x}^{n} - Y_{x}^{1}}} \right)}},{k \leq 1000},$where Y_(x) ¹ indicates the brightness values of the partial pixels ofthe frame numbered 1, Y_(x) ^(n) indicates the brightness values of thepartial pixels of the frame numbered N, and k indicates a number of thepartial pixels.